Diesel engine fuel injection valves usually have a housing in which a needle valve is slideably located, the needle valve being held in closed condition under a closing force. The closing force may be obtained by springs, or by magnets. When a pressure pulse is applied to a fuel inlet of the valve, the needle element is lifted off the valve seat, counter the closing force, and fuel can be injected to the combustion chamber of a diesel engine.
The closing force usually is provided by a helical spring which, as well known, has an essentially linearly rising force-deflection characteristic. It has been proposed to change such a linear characteristic by adding a magnet in the valve structure so that a second operating force is applied thereto, the second operating force, provided by the magnet, having a characteristic that the closing force drops rapidly with distance of the valve element from the valve seat, so that the characteristic of the second closing force is, with respect to the movement of the valve element of the valve seat is first rapidly dropping and then flattening out. The composite force acting on the valve element, typically a valve needle, due to the interaction of the force derived from the helical spring and from the magnet will have a characteristic which, from a substantial initial value first drops rapidly and then, in a second half of the total movement of the valve needle away from the valve seat, rises essentially linearly to a final value, in essence in accordance with the characteristic of the helical spring. The final closing force value--to be overcome by the fluid pressure is, however, smaller than the initial closing force, which is formed by the combination of the magnet and the spring.
To have a different characteristic, is to have the closing force rise progressively up to, and, for some applications even exceeding the original closing force level when the valve element has reached its final open position. Such a characteristic permits closing of the valve even if the pressure of the fluid within the valve still pertains, so that the valve element can be returned to the valve seat rapidly and without delay. This is highly desirable to prevent carbonization, and carbon deposits at the nozzle opening. Upon opening of the valve, however, the rapidly decreasing force permits fast injection, which causes a rapid increase in quantity of fuel being injected--with respect to time, or opening distance of the valve element. This is desirable in operation of the engine, since the fuel consumption of the engine, for a given power output is thereby reduced.
It is extremely difficult to obtain a distance versus force characteristic as described when using spring elements without increasing the diameter of the valve housing, or of the overall valve structure. The diameter of the valve housing, however, should remain within standardized limits to permit association of the valve with existing engine structures, and to remain within available space on the engine cylinder blocks.